Process for plasticizing lignocellulosic materials



Patented Oct. 4 6, 1942 1 PROCESS FOR PLASTICIZING LIGNO- CELLULOSIC MATERIALS William Karl Loughborough, Madison, Wis, assignor to Henry A. Wallace, as Secretary of Agriculture of the United States of America. and his successors in oflice No Drawing. Application July 15, 1940, Serial No. 345,676

(Granted under the. act of March a, 1883, as amended April so, 1928; :70 o. G. 757) This application is made under the act of March 3, 1883, as amended by the act of April I 30, 1928, and the invention herein described and claimed, if patented, may be manufactured and used by or for the Government of the United States of America for governmental purposes, without the payment to me of any royalty thereon.

My invention relates to processes for plasticizing wood and other ligno-cellulosic materials and to products made from said plasticized materials.

A principal object of my invention is to produce solid compressed wood of great density and strength. a

A further object of my invention is to produce molded articles from comminuted wood.

A further object of my invention is to produce plywood and laminated compressed wood of great density and strength.

A further object of my invention is to produce wood which can be readily deformed when hot and dry; also when hot and wet.

Other objects of my invention include the proresins are produced by the addition of some extraneous chemical, usually. formaldehyde, which condenses with the urea and/or thiourea.

I am also aware that urea and other chemicals have been,used in the chemical seasoning of wood. However, the purposes, methods employed, and results secured-are all entirely difierent from my invention. In chemical seasoning, it is essential that only the surface layers of the piece of wood to be dried are impregnated with the chemical, whereas in my invention the entire volume of the wood, or other ligno-cellulosic material, should be impregnated. In chemical seasoning, the vapor-pressure characteristics of the aqueous solutions of the chemical and the anti-shrink properties of the dry chemical are of paramount importance; whereas in my invention the ability of the aqueous solution of the chemical to soften the fiber, particularly the ligneous portion thereof, is the most essential property. In chemical seasoning it is desired to maintain the green size and form of the piece of wood insofar as possible;

whereas in my invention an important object is to modify the original form.

I have discovered that urea, NHzCONHz, and thiourea, NHzCSNHs, separately or in combination, have a marked softening efl'ect, at elevated temperatures, upon lignin, cellulose, ligno-cellulose, and other natural constituents of trees and plants, and that this effect may be applied to produce new and useful results. Most of the experimental work upon which this application is based was done upon wood, wood lignin, and wood cellulose; but my invention is equally applicable to lignin, cellulose, and ligno-cellulose of other origin, such as vegetable fibrous materials. The discovered effects vary widely in intensity among the various ligneous and cellulosic materials of differing origins: therefore, successful application of my invention will depend in large measure upon the proper selection of materials, and successful utilization of any given material will likewise depend upon the proper selection of a suitable treating or conversion method. In general, the effects are most intense in the case of ligneous materials.

For brevity and clarity, whenever the word ligno-cellulose is used in this specification and in the claims, it is to be understood that I in clude in its definition and meaning both lignin and cellulose, separately, as well as in combination, and also materials containing both lignin and cellulose and other natural constituents of trees and plants in varying proportions; and mixtures thereof. I have in mind particularly trees and plants of all kinds.

It is not possible, in the present state of the art, to state with complete certainty exactly what chemical reactions take place and what physical phenomena manifest themselves in the practice of my invention; 'but it appears now that most of the results secured support the hypothesis that the principal effective action is one of softening. However, it is probable, especially in the case of certain materials, such as oak wood for instance, which contain unusually large percentages of materials which are neither cellulosic nor ligneous, that a certain amount of condensation and resinification does take place during portions of some of the processes'contemplated in my invention. But my invention is not primarily concerned with the formation of urea and/or thiourea resins through the condensation of the urear and/or thiourea with some extraneous chemical, other than cellulosic or ligneous material, added to the-mass.

, In previous experimental work I discovered that green wood may be readily impregnated with urea and/or thiourea by immersion in an aqueous solution of said chemical. The action is principally one of diffusion and it goes forward at atmospheric temperature and pressure. It may be hastened by the use of elevated temperature, but increased pressure has only a very small accelerating effect. Dry wood can also be impregnated in this manner, but the process is much slower than with green wood. Other methods of impregnation may also be used in the practice of my invention.

It is recognized that certain properties of the finished products, such as hygroscopicity, fire-resistance, decay-resistance, resistance to insect attack, weather-resistance, may be modified by the addition of various well-known chemicals-principally water-soluble chemicals. My invention contemplates the use of such chemicals in addition to urea and/or thiourea.

If wood be impregnated with urea and/ or thiourea and then dried, it will become so soft and flexible when heated in the dry condition that it may very easily be bent, twisted or otherwise deformed. Upon cooling, the wood resumes its original strength properties. If the heating be done at temperatures in the neighborhood of 200 F., repeated subsequent heatings will produce softening effects similar to those produced by the first heating: but if the impregnated wood be subjected to temperatures in the neighborhood of 300 F., the ability to soften under subsequent heating is lost. If desired, said chemical may be leached out of the deformed wood, by immersion or otherwise. The properties of the wood, after leaching, will be similar to those of untreated wood.

My invention contemplates a method of treating and bending or otherwise deforming wood which consists essentially of the following steps: impregnate the wood with urea and/or thiourea; dry the wood; heat the wood; bond or otherwise deform the hot dry wood to the required shape; cool the wood while it is being held in the required shape. In a modified embodiment of my invention, the hot dry wood, after having been given the required shape and while being held in this shape, is heated to a temperature higher than that to which it had been previously heated, and then cooled. Once cool, the restraining means may be removed.

In another modified embodiment of my invention the deformed and cooled wood, while still under restraint, is immersed in water and the chemical allowed to leach out completely or in part. After leaching, the wet wood is dried and the restraining means removed.

In similar manner, wood impregnated with urea and/or thiourea will be so soft when hot and wet that it may be bent, twisted, or otherwise deformed very easily.

My invention contemplates a method of treating and bending or otherwise deforming wood which consists essentially of the following steps: impregnate the wood with urea and/ or thiourea; heat the wet impregnated wood to a temperature of about 150 F. to 225 F.; bend or otherwise deform the hot wet wood to the required shape; dry the wood while it is being held in said required shape. When dry, the wood may be removed from the restraining means.

In a modified form of my invention, the dry wood, before removal from the restraining means, is heated to a temperature well above 212 F., and then cooled. The purpose of this final high-temperature treatment is to reduce the thermo-plastic properties of the impregnated wood and to produce a thermo-setting effect.

In another modified. form of my invention the deformed wet wood, while still under restraint, is

immersed in water and the chemical allowed to I leach out completely, or in part. After leaching,

the wet wood is dried and the restraining means removed.

Dry wood previously impregnated with urea and/or thiourea may be compressed and/or molded, when heated, to form a material of great density which acquires remarkable strength properties upon cooling. My process is not critical as to the details of its operation. It is important, however, that a comparatively large amount of urea and/or thiourea be impregnated into the wood. I have found that oak which has been impregnated with 25 percent of its weight of urea and/or thiourea, dry basis, is suitable. This quantity of said chemical may be got into one-half-inch green oak boards by soaking them in an aqueous solution, containing one pound of urea and/or thiourea for each pound of water, for a period of 3 days at a temperature of F. In general, concentrated solutions are desirable, and temperatures ranging from room temperatures to about F. have given good results. After said soaking period the wood should be dried to a moisture content of about 2 to 10 percent. This drying can be done by any well known and suitable method. The dry wood is now ready for the molding or compressing process. It is possible to compress boards between flat platens, without the use of molds, but the results are not so'satisfactory as those secured with molds. The dry wood is cut to suitable size and shape and placed in the mold and. heated, by steam-jacketed platens, or otherwise, to a tem-' perature of about 280 F. to 360 F. It is then subjected to a pressure ranging from about 150 to 4000 pounds per square inch. Usually a pressure of about 3000 pounds per square inch is preferred. After completing the heating and pressing, and before removal, the mold and the wood must be cooled. The heating, pressing, and cooling operations will usually require less than an hour. The density of the finished product may be as high as 1.3. In addition to solid wood, my invention contemplates the use of laminated wood and plywood. Under the conditions existing dur- I ing the pressing operation, many species of wood become self-gluing and the finished product has the appearance and behavior of a single solid mass, whether originally of a single piece or of several pieces. For best results the wood must be very dry, ranging in moisture content from 2 to S'percent.

I have found, however, that, under certain conditions of moisture and chemical concentration, certain species of wood such as spruce for instance, do not become self-gluing under usual pressing conditions; especially when said wood is impregnated with comparatively small amounts of chemical. Under these conditions it becomes desirable to augment or fortify the comparatively slight self-gluing characteristics of the impregnated wood by means of suitable additional glue. In general, any of the well-known hot-press resin glues may be added for this purpose. The most convenient way to add said glue is in sheet form; one sheet of glue film between each pair of adjacent sheets of wood.

Wood sawdust, comminuted wood, and similar ligno-cellulosic materials, when impregnated with by. any suitable molding methods. My invention contemplates the production of molded articles composed largely or wholly or ligno-cellulosic materials combined with urear and/or thiourea. The individual steps in the. process are simple and non-critical and may each be executed in various well known ways. The properties of the finished product can be controlled over wide ranges by proper selection of materials anddetails of processing; all in accordance with the skill and knowledge of those versed in the art. The use of fillers of various materials is contemplated. The first step in the process is the impregnation of the ligno-cellulosic material with urea and/or thiourea. This may be done in any customary manner such asby soaking in an aqueous solution of said urea and/or thiourea.

The amount of said urea and/or thiourea required in the finished molding material will vary widely depending upon surrounding conditions. Good results have been secured with molding materials containing said urea and/or thiourea within the range of from percent to 70 percent of the weight of the finished molding material, dry basis. After a suificient amount of urea and/or thiourea has been absorbed, the next step is to dry the mass; an operation which maysbe performed in'any well 'known manner suited to .the specific nature of the materials. The simplest way is to drain or filter excess liquid off and then spread the mass in thin layof water. The sheets are then removed from the solution and'dried toa moisture content of about 5 percent, by any wellknown and suitable method. The sheets are then'assembled in a mold in a hot press in stacks of suitable thickness with the grain of adjacent plieso'riented under a pressure of about 4000 pounds persquare inch of sheet area. The temperature of the press platens is then raised as rapidly as .convenient to about 360 F., which temperature is main- The tained for a period of about 10 minutes. I press is then allowed to cool. After cooling the press may be opened and the compressed-wood boards removed. V 1

III. To produce molded articles: Finely ground oak sawdust is soaked at room temperature for a period of about one day in an aqueous solution of urea and/or thiourea, the concentration of said solution being adjusted so that there will be 20' pounds of said chemical, when dry, for each 100 pounds of dry sawdust. The amount of water used is not critical, but good results have been obtained by using. one pound of water for each pound of dry sawdust. After the soaking period the solution is drainedofl, and the sawwithrespect to'each ,other according to whatever-- arrangement is desired. The press is then closed dust dried to a moisture content of about 5 per- I be used as suchby any suitable method. One

method which has produced satisfactory results is as follows: Weigh out and place in a steamers on trays and dry in a. warm room or simple 7 dry kiln. Sawdust and similar material can be advantageously ground to a powder for molding intricate forms of finished product.

Specific examples of how my invention may be practiced follow:

I. To produce bent wood parts about an inch square suitable for furniture and other uses:

Green oak squares of suitable length are soaked at room temperature for a period of about 3 days in an aqueous solution of urea and/or thiourea, containing about one part by weight of said chemical, when dry, for each part of water. The squares are then removed from the solution and dried to a moisture content of about 5 percent, by any well knownmethod. The squares are then heated to a temperature of about 200 F.;

and, while, hot, are bent over suitable forms, to any desired shape, by any customary method. The squares are then cooled and must be suitably restrained in the bent shape till they have cooled off. After cooling, they may be removed from the forms. If it be desired to make the bent squares immune to the softening effect of hightemperature, they may be set by heating them, -'while in the forms, to a temperature of about 340 F., and maintaining them at this temperature for approximately a half hour. If it be desired to remove said chemical from said bent weight of said chemical, when dry, for each part heated mold the proper amount of dry sawdust. Assemble the mold and subject it to a pressure the equivalent of about 4000 pounds per square inch of mold area for a period of about 30 minutes. During about 10 minutes of said period a mold temperature of about 360 F. should be maintained. At the end of said period allow the mold to cool, under'the elevated pressure, to a temperature at least as low as F. After cooling, the pressure may be released, the mold opened, and the finished article removed.

IV. To produce glued laminated compressed wood and plywood boards: Sheets of green spruce veneer about 3% inch thick are soaked at a temperature of about F; for a period of about 24 hours in an aqueous solution of urea and/or thiourea, containing one pound of said chemical, when dry, for every two pounds of water. sheets are then removed and hung up to dry preferably in an open shed. After drying, they are assembled into layered stacks, for pressing. Assuming that 3-sheet plywood is to be made, the procedure recommended is as follows: a sheet of veneer is laid on the assembly boards, with the grain in the North and South direction; a sheet of resin glue film is then laid down onto the sheet of veneer; a second sheet of veneer is then laid down onto the stack, with its grain in the East and West direction; a second sheet of resin glue film is then laid down onto the stack; a third sheet of veneer is then laid'down onto the stack,

The'

with the grain in the North and South direction. I

utes, completing the process, after which said board may be removed from the press. The same general procedure may be followed in building up boards of any desired thickness and any desired number of layers. The angle of grain between adjacent layers may be as desired; for plywood it is customarily 90 as in the specific example Just described; for laminated wood it is customarily and for wheel and gear blanks, it may well be from 30 to 60.

V. .To produce bent wood parts about an inch square from green wood: Green spruce squares of suitable length are soaked at room temperature, for a period of about 3 days, in an aqueous solution of urea and/or thiourea, containin one part by weight of said chemical, when dry, for each part of water. The temperature of said solution is then raised to about 150 F., as rapidly as convenient. As soon as this temperature is attained, the squares are removed and while hot are bent to the required shape, by any customary method employing removable forms. The bent squares, retained on th forms, are then placed in a dry kiln, of the type commonly used for drying lumber, and are dried to a moisture content of I about 5 percent. This drying must be done carefully; a temperature of about 140 F. is suitable, and the relative humidity should range from about 75 percent at the start of the drying operation to approximately percent at the finish. When the squares have become dry, the temperature of the kiln is elevated to about 320 F. and maintained at this level for about an hour. The kiln is then cooled to about 140 F., completing the process. If it be desired to remove said chemical from the bent squares they may, upon removal from the bending machine and while still on the forms, be immersed in water and allowed to leach for several days. After completion of the leaching, the bent, extracted squares may be kiln dried as specified.

VI. To produce a 3-layer compressed-wood laminated article of high density and strength: Three sheets of green oak veneer 5; inch thick are soaked for a period of about six hours at room temperature in an aqueous solution of urea and/ or thiourea containing one part by weight of said chemical, when dry, for each two parts of water.

The sheets are then removed .and hung in a dry kiln of the type commonly used for drying lumber. The temperature within this kiln should b maintained at about 140 F., and the relative humidity should be about 20 percent. After said sheets have become dry they are removed from the kiln. They are trimmed to the proper size to fit the mold, are stacked together with the direction of grain of all parallel, and placed in the mold.

The mold is then closed. and heated as rapidly as expedient to a temperature of about 360 F.

This temperature is maintained for approximately 10 minutes. Throughout the pressure treatment a pressure of about 1000 to 4000 pounds per square inch is maintained. The mold is then cooled under pressure and opened when cooled, completing the process.

Single sheets of woodand stacks of sheets with the angle of grain of adjacent layers arranged in any desired pattern may be molded in similar manner.

Having thus described my invention, what I claim for Letters Patent is:

1. A process comprising impregnating wood with an aqueous solution or one of the group of chemicals consisting of urea, thiourea, and urea and thiourea combined; thence drying said impregnated wood; thence heating said dry impregnated wood to at least 1 thence deforming the hot dry impregnated wood; and thence cooling the deformed wood.

2. A process comprising impregnating wood with an aqueous solution of on of the group of chemicals consisting of urea, thiourea, and urea and thiourea combined; 'thence drying said impregnated wood; thence heating said dry impregnated wood to at least 150 F.; thence deforming the hot dry impregnated wood; thence extracting said chemical from the deformed wood; and thence drying the deformed wood.

3. A process comprising impregnating wood with an aqueous solution of one of the group of chemicals consisting of urea, thiourea, and urea and thiourea combined; thence drying sai impregnated wood; thence heating said dry impregnated wood to at least 150 F.; thence deforming the hot dry impregnated wood; thence heating said deformed wood to at least 200 F.; and thence cooling the deformed wood.

4. A process comprising heating and impregnating wood in an aqueous solution of on of the group of chemicals consisting of urea, thiourea, 'and urea and thiourea combined at a temperature of at least 150 F.; thence deforming the hot wet impregnated wood; thence cooling and drying said deformed hot wet impregnated wood; thence heating said deformed wood to at least 200 F.; and thence cooling the deformed wood.

5. A process for making compressed wood which comprises soaking wood in an aqueous solution of one of the group of chemicals consisting. of urea, thiourea, and urea and thiourea combined; thence drying said wood; thence compressing said dried wood at elevated temperature; and thence cooling said dried compressed wood under pressure.

6. A process for making compressed impregnated laminated wood which comprises assembling a stack of sheets of wood impregnated with one of the group of chemicals consisting of urea, thiourea, and urea and thiourea combined together with sheets of resin glue film between adjacent sheets of said wood; thence pressing said stack at elevated temperature; and thence cooling said stack under pressure.

WILLIAM KARL LOUGHBOROUGH. 

